CARBONIFEROUS BIOCHRONOSTRATIGRAPHY OF THE AMAZONAS BASIN, BRAZIL, BASED ON CONODONTS

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1 CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) CARBONIFEROUS BIOCHRONOSTRATIGRAPHY OF THE AMAZONAS BASIN, BRAZIL, BASED ON CONODONTS LEMOS, V.B. 1 and SCOMAZZON, A.K. 1 (1) Institute of Geosciences, Department of Paleontology and Stratigraphy, Federal University of Rio Grande do Sul - UFRGS, Bento Gonçalves Avenue, 9500, Agronomia District, Porto Alegre / RS, , Brazil. ( valesca.lemos@ufrgs.br, scoma@vortex.ufrgs.br) ABSTRACT The Carboniferous conodont biochronostratigraphy of the Amazonas Basin, in the eastern Amazonas region of northern Brazil, comprises eight biozones. Three of these consist of assemblage zones: Neognathodus symmetricus / Rhachistognathus muricatus Zone (Morrowan), Diplognathodus orphanus / Diplognathodus coloradoensis Zone (Atokan), and Idiognathodus claviformis / Idiognathodus incurvus Zone (attributed to the Early Desmoinesian). The oldest zone refers to the Monte Alegre Formation and the lower part of the Formation. The second zone corresponds to the middle and upper parts of the Formation. The third and youngest zone corresponds to the lower part of Nova Olinda Formation. Two assemblage zones have been also established: the Neognathodus symmetricus / Rhachistognathus muricatus Zone (Morrowan) and Diplognathodus spp. Zone (Atokan). Lastly, three local interval zones are recognized: Idiognathodus sinuosus / Rhachistognathus muricatus Zone (Late Morrowan); Idiognathodus klapperi / Streptognathodus parvus Zone (Late Morrowan Atokan); and Idiognathodus claviformis / Idiognathodus incurvus Zone (Early Desmoinesian). INTRODUCTION Conodonts have been recovered from wells and outcrops in the Carboniferous sedimentary rocks of the Amazonas Basin, in northern Brazil (Fig. 1). Lithologically most the section covered by this study consists of calcarenites, calcilutites, sandstones and shales. It comprises the Monte Alegre Formation, Morrowan in age, and the overlying and Nova Olinda Formations of Atokan and early Desmoinesian age respectively. Twelve core samples have been obtained from the Monte Alegre Formation, 94 from the Formation, and 21 from the Nova Olinda Formation. All 127 samples derive from 18 scattered PETROBRAS wells, plus 35 outcrop samples from the Formation. Two biostratigraphic zones were previously set up for the Carboniferous sections of the Amazonas Basin (Lemos, 1990). These are the Neognathodus symmetricus / Rhachistognathus muricatus Zone and Diplognathodus coloradoensis / Diplognathodus orphanus Zone. The third zone, formerly named Streptognathodus elongatus / Idiognathodus ellisoni, is now redesignated Idiognathodus claviformis / Idiognathodus incurvus Zone (Fig. 2) (Lemos & Scomazzon, in prep). The main purpose of this study is to briefly discuss the Carboniferous conodont biochronostratigraphy of the Amazon Basin in the light of our new results. Previous conodont studies in the Carboniferous of the Amazonas Basin include those by Tengan et al. (1976) and Rocha-Campos et al. (1988). However, none of these attempted to correlate the regional assemblages with coeval North American faunas. The Carboniferous geology of the Amazonas Basin has been treated by Mendes (1959, 1961, 1967), Caputo et al. (1971), Daemon & Contreiras (1971), Carozzi et al. (1973), Santos et al. (1975), Carozzi (1979), Caputo (1984), Della Favera & Possato (1985), Quadros (1986), Altiner & Savini (1996), Silva (1996), Loboziak et al. (1997), Melo et al. (1999), and Playford & Dino (2000a, b). These authors also discuss the paleoecology and biostratigraphy based on palynomorphs and fusulinids, as well as the cyclicity so well marked in Carboniferous marine deposits of that basin. After 90s the study of Amazon Basin conodonts became systematic. Ever since an effort has been made to define regional biozones based on conodonts, because of their value as excellent biostratigraphic tools, attested by innumerous scientific publications. The main problem being faced nowadays concerns the reduced number of researchers really dedicated to conodont studies in Brazil, and the formidable amount of sedimentary sections in wells and outcrops that remain to be examined. MATERIALS This work is based on the analysis of eighteen wells and four outcrops, abundant in carbonate and some shales. The samples cover rock units ranging in age from late Morrowan to early Desmoinesian. Standard preparation techniques were used. Samples were crushed into pieces of 1 to 3 cm in diameter, then treated with glacial acetic acid (10-15%). The residues thus obtained were washed through a mm standard sieve, from which the conodonts were picked and mounted on microslides to be classified. BIOCHRONOSTRATIGRAPHIC RESULTS The conodont zones first established by Lemos (1990) are now reviewed in the light of new research. 131

2 CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) When they are compared to Altiner & Savini (1996) foraminiferal zones the results are quite compatible. Morrowan and Morrowan/Atokan ages assigned on the basis of foraminifera to the Monte Alegre and Formations, respectively, agree with the results of our study. However, Lemos (1990) third and youngest conodont zone, the Streptognathodus elongatus / Idiognathodus ellisoni Zone, originally dated earlier Virgilian and reviewed herein, is now assigned to the Desmoinesian and renamed Idiognathodus claviformis / Idiognathodus incurvus. Neis (1996) erected two Carboniferous biozones: the Rhachistognathus muricatus / Idiognathoides ouachitenses Zone, late Morrowan in age, situated in the base of Formation, and the Diplognathodus orphanus / Diplognathodus coloradoensis Zone of Atokan age, within the middle part of the same formation. These results were based on the study of six PETROBRAS wells in the Amazonas Basin. Scomazzon (1999), investigating four limestone sections cropping out in the southern margin of the Amazonas Basin, established three local interval zones: Idiognathodus sinuosus / Rhachistognathus muricatus Zone, late Morrowan; Idiognathodus klapperi / Streptognathodus parvus Zone, late Morrowan Atokan; and Idiognathodus claviformis / Idiognathodus incurvus Zone, early Desmoinesian. The biochronostratigraphic results newly obtained permit to recognize the Morrowan/Atokan boundary within the middle portion of the Formation in wells 2-CA-1-AM (Fig. 3) and 1-UI-2-AM (Fig.4). The Atokan/Desmoinesian boundary was documented, in well 2-PE-1-AM (Fig. 5), within a lithological interval that comprises the top of the Formation and the lowest part of the overlying Nova Olinda Formation. The Neognathodus symmetricus / Rhachistognathus muricatus Zone, the lowest conodont zone, Morrowan in age, encompasses the Monte Alegre Formation and lower part of the Formation in well 2-CA-1-AM. This zone is now being refined by means of the study of the genus Rhachistognathus and its species. In North America the same genus is known to range from late Mississipian through middle Pennsylvanian. Four species of Rhachistognathus have been recorded in the Amazonas Basin: Rhachistognathus muricatus (1-AM-2-AM # 3); Rhachistognathus prolixus (1-UI-2-AM #3); Rhachistognathus primus (2-PE-2-AM #2; 1-UI-2-AM #2) and Rhachistognathus minutus minutus (1-UI-2-AM #3). According to the range charts of Baesemann & Lane (1985) this assemblage of Rhachistognathus spp. would correlate with the Idiognathoides sinuatus / Rhachistognathus minutus and Neognathodus symmetricus Zones of early Morrowan age. The second unit, Diplognathodus orphanus / Diplognathodus coloradoensis Zone, of Atokan age, is found in the Formation in well 1-UI-2-AM. The third and youngest biozone, Idiognathodus claviformis / Idiognathodus incurvus, occurs near the base of the Nova Olinda Formation in the well PE-1- AM, and is attributed to the early Desmoinesian. The marine sedimentation during the Carboniferous was controlled by glacioeustatic sea-level oscillations (Wanless & Shepard, 1936; Heckel et al., 1980; Veevers & Powell, 1987). In the Amazonas Basin the Carboniferous strata were formed in onlap, with non depositional hiatuses over higher parts of the basin separating each transgressive - regressive depositional sequence. Conodonts were widespread in the Carboniferous Amazonas seas during each transgressive phase, when salinity was favourable. The sedimentation was characterized by black shales, carbonates and evaporites. Most of the conodonts are found in carbonates, but some also occur in the shales. The biochronostratigraphic studies allow us to conclude that the Monte Alegre Formation was deposited during the Morrowan. The Formation has its lowest section in subsurface still in the Morrowan, but higher up the unit ranges into the Atokan. The Nova Olinda Formation, at least in one of the studied wells, bears a conodont assemblage of Desmoinesian age. REFERENCES Altiner, D. & Savini, R., Pennsylvanian foraminifera and biostratigraphy of the Amazonas and Solimões Basins (North Brazil). Rev. Paléobiol., 14(2): Baesemann, J.F. & Lane, H.R., Taxonomy and Evolution of the Genus Rhachistognathus Dunn (Conodonta; Late Mississippian to Early Middle Pennsylvanian). Courier Forsch. Senck., 74: Caputo, M.V., Stratigraphy, tectonics, paleoclimatology and paleogeography of northern basins of Brazil. Ph.D. Univ. California, Santa Barbara. Caputo, M.V. et al., Litoestratigrafia da Bacia do Amazonas. Belém, PETROBRAS, 21 pp. (Internal Report, 644-A). Carozzi, A.V., Petroleum exploration of the Paleozoic clastics of the Middle Amazon Basin. J. Petrol. Geol., 2(1): Carozzi, A.V. et al., Ambientes deposicionais e evolução tectono-sedimentar da seção clástica Paleozóica da Bacia do Médio Amazonas. In: Congr. Brasil. Geol., 27., Aracaju, Anais... Aracaju, SBG, v. 3, p Cunha, P.R.C.; Gonzaga, F.G.; Coutinho, L.F.C. & Feijó, F.J., Bacia do Amazonas. Bol. Geoci. Petrobras, 8 (1): Daemon, R.F. & Contreiras, C.J.A., Zoneamento Palinológico da Bacia do Amazonas. In: Congresso Brasileiro de Geologia, 25., São Paulo, SP, Anais... São Paulo: SBG., p Della Favera, J.C. & Possato, S., The use of depositional sequences as a tool for oil exploration in Brazilian sedimentary basins. In: Geological Congress of the Middle East, 2, Bagdad. Proceed., v. 1, p

3 CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Heckel, P.H. et al., Glacial eustatic model for black-shale-bearing Pennsylvanian cyclothems of Midcontinent and Illinois. In: 10 th Ann. SEPM (Society of Economic Paleontologists and Mineralogists) Great Lakes Section. Univ. Urbana Field Conference Guide Book, p Lemos, V.B., Assembléia de conodontes do Carbonífero da Bacia do Amazonas. Ph.D. Thesis, Univ. Fed. Rio Grande do Sul, Porto Alegre, 259 p. Lemos, V.B. & Scomazzon, A.K., in press. Conodont Biostratigraphy of Pennsylvanian in the Amazonas Basin, Brazil. Paleobios. Loboziak, S. et al., Earliest taeniate bisaccates from the Amazon Basin are not older than Westphalian. Geobios, 30 (4): Melo, J.H.G. et al., Latest Devonian to early Late Carboniferous biostratigraphy of northern Brazil: an update. Bull. Centres Rech. Expl.-Prod. Elf Aquitaine, 22: Mendes, J.C., Chonetacea e Productacea Carboníferos da Amazônia. Bol. Fac. Filos., Ci. Letras, USP. Geologia, 17(236): Mendes, J.C., Notas suplementares sobre os brachiopodes carboníferos da Amazônia. Bol. Soc. Brasil. Geol., 10(1): Mendes, J.C., Paleobiota carboníferos da Amazônia. [sic] In: Simpósio sobre a Biota Amazônica, Belém. Atas... Rio de Janeiro, p Neis, P.A., Resultados biocronoestratigráficos das associações de conodontes da Formação, Carbonífero Superior (Pensilvaniano), da Bacia do Amazonas. M.Sc. Dissert., Univ. Fed. Rio Grande do Sul, Porto Alegre, 138 p. Playford, G. & Dino, R Palynostratigraphy of upper Paleozoic strata (Tapajós Group), Amazonas Basin, Brazil: Part One. Palaeontographica, 255 (1-3): Playford, G. & Dino, R Palynostratigraphy of upper Paleozoic strata (Tapajós Group), Amazonas Basin, Brazil: Part Two. Palaeontographica, 255 (4-6): Quadros, L.P., Ocorrência de microfósseis (Acritarcha) ordovicianos na sub-bacia do Alto Amazonas, Brasil. In: Congresso Argentino de Paleontología y Bioestratigrafía, Mendoza. Actas... v. 1, p Rocha-Campos, A.C. et al., Índices de Alteração de Cor (IAC) de conodontes do Carbonífero da Bacia do Amazonas. In: Congr. Argent. Paleont. y Bioestrat., 4., Mendonza, Argentina, Resumenes... Mendonza, v. 3, p Santos, D.B. et al., Geologia. In: Brasil. Projeto Radambrasil. Folha SB. 21 Tapajós. Rio de Janeiro, DNPM, p Scomazzon, A.K., Refinamento bioestratigráfico com base em conodontes, no Pensilvaniano da Bacia do Amazonas Região do Tapajós. M.Sc. Dissert., Univ. Fed. Rio Grande do Sul, Porto Alegre, 142 p. Silva, O.B., Ciclicidade sedimentar no Pensilvaniano da Bacia do Amazonas e o controle dos ciclos de sedimentação na distribuição estratigráfica dos conodontes, fusulinídeos e palinomorfos. Ph.D. Thesis, Univ. Fed. Rio Grande do Sul, Porto Alegre, 331 pp Tengan, C. et al., Conodontes carboníferos do poço FB-ST-1-AM Bacia do Amazonas, Brasil. In: Congr. Brasil. Geol., 29, Ouro Preto, Resumos... Ouro Preto, SBG, p Veevers, J.J. & Powell, C.McA., Late Paleozoic glacial episodes in Gondwanaland reflected in transgressive-regressive depositional sequences in Euroamerica. Geol. Soc. Amer. Bull., 98: Wanless, H.R. & Shepard, F.P., Sea level and climatic changes related to late Paleozoic cycles. Geol. Soc. Amer. Bull., 47:

4 CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Juruá river Negro river Solimões river Purus river AI-2 AI-1 PE-2 LM-1 AM-2 MD-1 IC-1 CA-1 CM-2 PE-1 Amazonas river MI-2 UI-2 Tapajós river CM-3 MA-1 CP-2 Outcrops Xingú river Manaus city Venezuela Colombia Guianas Shield 0 0 Equador Iquitos Arc Acre Basin Solimões Basin Purus Arc Gurupa Arc Brazilian Shield 0 km Peru Bolivia BRAZIL Paraguay Chile Uruguay Argentina km Fig. 1 - Location Map of the Amazonas Basin, northern region of Brazil. In detail the bore holes and outcrops studied in this paper. AM-5, AM-7 and AM-9 are on the same place that AM-2 (modified from Lemos, 1990). 134

5 Geochrn. Stratigraphic Chart of Amazonas Basin Lithostratigraphy Time Unity (Ma) Period Gr. Fm. Andirá Lithology Tapajós 300 Nova Olinda Monte Alegre Tardi-Hercian Orogeny Eo-Hercynian Orogeny Famatinian Orogeny fluvialaeolic Faro Oriximiná Curiri Barreirinha Ererê CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Environment fluviallacustrine marine high salinity Sineclise Fluvial, Deltaic, Glacial Shallow marine Tectonic Evolution Depositional Sequence Sineclise C - P Sineclise D - C Urupadi I - Idiognathodus R - Rachistognathus Lemos & Scomazzon (2001) Lemos (1990) Neis (1996) N - Neognathodus D - Diplognathodus S - Streptognathodus Scomazzon (1999) Fig. 2 - Chronostratigraphic chart of the Amazonas Basin ( modified from Cunha et al., 1994 ). Siltstone Sandstone AND NOL ITA Id. ouachitensis MAL Id - Idiognathoides MAE Shale FAR ORI CUR BAR ERE MAN Carbonate Anhydrite Halite V Diamictite Diabase sills I. claviformis / I. incurvus I. claviformis / I. incurvus R - Rhachistognathus Morrowan Atokan Desmoinesian Pennsylvanian Mississipian Permian Carboniferous Devonian 135

6 Rachistognathus m uric atus Neognathodus sym m etric us Adetognathus lautus I Hindeodus m inutus Neognathodus bassleri Neognathodus m edadultim us CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Gamma-ray log Lithology CA-1-AM Bore Hole CONODONTS Cretac eous T1 Neognathodus roundyi diognathodus delicatus Diplognathodus coloradoensis Streptognathodus elegantulus Idiognathoides sinuatus Diplognathodus orphanus Idiognathodus m agnific us Idioprioniodus sp. 300 T2 T3 400 T4 500 T5 Atokan T6 Morrowan T7 LEGEND Monte Alegre 600 Carbonate Anhydrite Halite Shale Sandstone Fig. 3 - Gamma-ray log, lithology and conodonts distribution in the well CA-1-AM. 136

7 BORE HOLE CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Gamma-ray log Lithology UI-2-AM Nova Olinda CONODONTS T1 Idioprioniodus sp. Streptognathodus suberectus Neognathodus roundyi Streptognathodus parvus Neognathodus bassleri Idiognathoides sinuatus Idiognathodus delicatus Rachistognathus muricatus Adetognathus lautus Strepto gnathodus expansus Streptognathodus elegantulus Hindeodus minutus Diplognathodus orphanus Diplognathodus coloradoensis 200 T2 Atokan 250 Monte Alegre T3 Morrowan LEGEND Carbonate 500 Anhydrite Halite Shale Sandstone Fig. 4 - Gamma-ray log, lithology and conodonts distribution in the well UI-2-AM. 137

8 Bore Hole CORRELAÇÂO DE SEQÜÊNCIAS PALEOZÓICAS SUL-AMERICANAS (J.H.G. MELO & G.J.S. TERRA, eds.) Gamma-ray log Lithology PE-1-AM CONODONTS T2 Desmoinesian Nova Olinda 850 T3 900 T4 Atokan Monte Alegre LEGEND Carbonate Anhydrite Halite Shale Sandstone Fig. 5 - Gamma-ray log, lithology and conodonts distribution in the well PE-1-AM. 138